US7170545B2 - Method and apparatus for inserting variable audio delay to minimize latency in video conferencing - Google Patents
Method and apparatus for inserting variable audio delay to minimize latency in video conferencing Download PDFInfo
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- US7170545B2 US7170545B2 US10/832,547 US83254704A US7170545B2 US 7170545 B2 US7170545 B2 US 7170545B2 US 83254704 A US83254704 A US 83254704A US 7170545 B2 US7170545 B2 US 7170545B2
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L65/00—Network arrangements, protocols or services for supporting real-time applications in data packet communication
- H04L65/40—Support for services or applications
- H04L65/403—Arrangements for multi-party communication, e.g. for conferences
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L65/00—Network arrangements, protocols or services for supporting real-time applications in data packet communication
- H04L65/1066—Session management
- H04L65/1101—Session protocols
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L65/00—Network arrangements, protocols or services for supporting real-time applications in data packet communication
- H04L65/80—Responding to QoS
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N7/00—Television systems
- H04N7/14—Systems for two-way working
- H04N7/15—Conference systems
Definitions
- This invention relates to video conferencing, in particular relates to audio video synchronization in video conferencing and audio latency reduction.
- video and audio signals from one site are digitally compressed and then sent over conventional communication channels (such as ISDN, IP, etc.) to either a bridge, which will then send the signal on to multiple sites, or to a second site directly.
- video delay may be referred to as video delay.
- Common practice is to delay the audio to match this video latency in order to maintain synchronization, also called lip-sync. But it is important to note that audio does not have the same inherent delay as video, in part because the algorithms are designed to minimize delay, so this delay is only inserted to synchronize the audio with the video.
- the inherent delay in audio processing is usually much less. This inherent delay in audio processing is herein referred to as minimum delay.
- one important element is the natural back-and-forth conversational flow.
- 500 to 200 ms are added to synchronize the audio to the slower video, much of the efficiency of the conversation is lost. People will talk over each other, then stop, then talk over each other again. They may deliberately wait, knowing that this delay exists, and so produce a perception of arrogance where none actually exists.
- a variable audio delay is inserted into the audio stream according to the different situations during a video conferencing: a longer delay is inserted during a monologue, where only one participant is speaking; a short delay or no delay is inserted during an interactive discussion or influenced argument.
- the replay speed of the audio is varied between slower than normal speed to introduce delay or faster than normal speed to reduce delay.
- a digital speed adjustment is employed such that the audio pitch is not affected by the varied replay speed.
- FIG. 1 illustrates an analog sound wave
- FIG. 2 illustrates the video and audio signal replay related to playing time where no artificial delay is inserted to audio replay.
- FIG. 3 illustrates the video and audio signal replay related to playing time where an artificial delay is inserted to audio replay such that video and audio replay are synchronized, i.e. lip-synced.
- FIG. 4 illustrates the video and audio signal replay related to playing time and the switching between the interactive mode and the non-interactive mode.
- FIG. 5 illustrates the video and audio signal replay related to playing time where the audio replay is slowed down by 10% or 20% until audio and video are synchronized.
- FIG. 6 illustrates the video and audio signal replay related to playing time where audio replay is switched between different modes: delayed, normal, slowed for different conferencing conditions.
- FIG. 7 illustrates the video and audio signal replay related to playing time where audio replay is slowed-down/speeded-up/normal for different conferencing conditions.
- FIG. 8 illustrates the interaction of two audio channels and the Interaction Detector changing the audio replay between a monologue mode and an interactive mode.
- FIG. 9 illustrates a typical conferencing arrangement between camera, video display, microphones and speakers.
- FIG. 10 is a state diagram illustrating the conferencing system operating in various modes depending on the different conferencing conditions as illustrated in FIG. 6 .
- FIG. 11 is a state diagram illustrating the conferencing system operating in various modes depending on the different conferencing conditions as illustrated in FIG. 7 .
- a method of this invention is to vary the delay of the audio channel as a function of the degree of interaction or influencingness that is occurring, instead of keeping the audio delay constant.
- audio delay is cut to a minimum by this embodiment.
- the delay is allowed to increase sufficiently to synchronize audio to the video.
- FIG. 2 shows a relationship between audio and video signals when there is minimum audio delay.
- real time is shown on the horizontal axis
- replay time i.e. the time at which a signal (audio or video) exits the system
- a horizontal distance between two points at a same height is the time delay between the two points in replay.
- a line drawn at a 45-degree angle means that the replay speed is at a normal speed, i.e. the same speed as the signal is generated in real time.
- a line drawn at an angle less than 45 degrees means that the replay speed is slower than the normal speed, i.e. the speed the signal is generated in real time.
- the delay i.e. horizontal distance between this line and a 45-degree line
- a line drawn at an angle greater than 45 degrees means that the replay speed is faster than the normal speed.
- a 45-degree line having its lower end at the origin represents a signal with zero delay (replay time and real time are equal) being played at normal speed.
- a line drawn at a 45 degree angle, and having its lower end to the right of the origin, is the result of a signal that is experiencing some delay from the system (real time has gotten ahead of replay time or the replay time is behind the real time), but that is now being played at normal speed (but input and output speeds are the same, the signals are just offset).
- a line drawn at less than 45 degrees (more horizontal) shows real time proceeding more quickly than replay time, which is the result of a slowed replay.
- a line greater than 45 degrees, or more vertical, is of course the result of a speeded-up replay.
- FIG. 2 the audio and video replays are not in synch, i.e. video replay is about (DelaySync-DelayMin) seconds behind audio replay.
- video replay is about (DelaySync-DelayMin) seconds behind audio replay.
- the inserted delay is switched to DelaySync.
- FIG. 3 shows such a replay mode. In this mode, the additional delay is inserted into the audio replay such the total delay in audio replay is the same as the video replay. Thus the audio and video replays are synchronized.
- FIG. 1 shows a typical analog sound wave chart, showing air pressure of sound (or electrical signals driving a loud speaker) versus time.
- FIG. 1 may also represent the amplitude of sound on a record versus the length of the groove on the record.
- Algorithms have been available for many years to “stretch” and “squeeze” audio, or slow and speed the rate at which it is played. For a record player, that just means to rotate the record at a higher speed to “squeeze” the audio or at a slower speed to “stretch” the audio. Record players have been capable of this for a century, but at the cost of affecting pitch proportionally.
- FIG. 5 illustrates such speed changes.
- FIG. 5 illustrates the transition from DelayMin to DelaySync with 10% slower audio replay. Assuming the DelaySync is 2 seconds and DelayMin is 0.1 second, then it will take about 20 seconds for transition from DelayMin to DelaySync. In FIG. 5 , if the replay speed is slowed by 20%, then the time for the transition is cut from 20 seconds to about 10 seconds.
- FIG. 6 illustrates the audio and video replay relationship during a video conference when the audio replay mode is switched between interactive mode with minimum audio delay, monologue mode with audio-video synchronized and a transition mode in between.
- FIG. 10 illustrates the state changes corresponding to the embodiment as illustrated in FIG. 6 . All the time marks shown in FIG. 10 are the same time marks shown in FIG. 6 . In this example, for ease of discussion, assume the one-way video delay to be 2 seconds, and the audio delay to be 0.2 second.
- a second talker now interrupts at time mark 4 . Because he is an “interrupter”, his audio is sent with minimal delay (in this example, 0.2 second), but slowed down by 10%. This second talker speaks for 5 second. At the end of this time, his delay has built up to 0.2+0.5 seconds, or 0.7 seconds.
- the first talker now interrupts at time mark 5 . His audio is also sent with 200 ms delay, and slowed down by 10% because he is also now an “interrupter.”
- the second talker interrupts at time mark 6 and keeps talking. His audio is sent with 200 ms delay, but slowed down by 10%. As he continues talking, the effective delay between the words he speaks, and the words heard by the far end, increases because of the slowing that is inserted. When the effective delay reaches 2 seconds (the latency required to achieve lip-sync) after 18 seconds of his speech at time mark 7 , the slowing is removed and his speech returns to normal speed (the output speed is switched from 90% to 100%), but still affected by the 2-second delay that has built up.
- an interaction detector may be employed to examine the incoming audio and video information, in a manner to be described below and illustrated in FIG. 10 , and uses the resultant information to select one of three audio streams to be output: minimal delay, a stream slowed by 10%, or 2-second fixed delay.
- delay is normally set (while speech is not interactive) for 2 seconds so the switch selects the two-second delay.
- the switch is changed to select minimal delay, which is actually 0.2 second in the above example. This results in a loss of about 2 seconds of audio, but this is not very noticeable since this has occurred during an interruption anyway.
- the interaction detector sets the switch to the slowdown mode in the variable speed channel, e.g.
- the same technique is refined by providing a second variable-speed channel. All time marks in FIG. 11 are the same as in FIG. 7 .
- interaction is dealt with by speeding the channel up by 20% until the 2-second delay is eliminated, rather than by immediately switching to a zero-delay, as in the previous example. In this way, the 2 seconds of speech that was lost previously is preserved, but played very quickly.
- the second speaker interrupts. Rather than stop the first speaker immediately and discard the last two seconds of his speech, in this embodiment, the replay of the first speaker's speech continues until it is finished at time mark 4 ′, but at a faster speed, e.g.
- the second speaker's speech is buffered, and replayed after the first speaker's speech finishes at time mark 4 ′.
- the second speaker's speech is also replayed at a higher speed, e.g. 120% of normal speed, until the audio delay reaches the minimum delay at 4 ′′.
- the first speaker interrupts at time mark 5 .
- the second speaker's speech is stopped and the first speaker's speech is replayed with minimum delay. Since the second speaker's speech is replayed with minimum delay, there is minimal amount of his speech waiting in the buffer for replay and virtually nothing is lost during the switch between the second speaker to the first speaker.
- FIG. 11 illustrates the mode changes of the audio replay modes for the embodiment illustrated in FIG. 7 .
- the first and the second transition modes are transient modes during which the audio delay is increased (decreased) from DelayMin (DelaySync) to DelaySync (DelayMin). They are not steady state modes. Only the interactive mode and the non-interactive mode are steady state modes.
- an interactivity detector may be employed to assist the transition between different modes.
- the interactivity detector is to detect when audio has become more interactive, and to use this information to signal or control functions. This may be conveniently done by monitoring incoming audio.
- the most common means of performing this function is called a “Voice Activity Detector”, or “VAD”, which is commonly used in telephone and speech processing systems.
- VAD Voice Activity Detector
- a VAD works as follows. When the amplitude of an audio first rises above some threshold, as in the initiation of speech, “interactivity” can be deduced. When it has remained above that threshold without another participant having similarly exceeded such a threshold, it can be concluded that the speech is no longer “interactive” but has become one-sided or monologue.
- FIG. 8 illustrates an interactivity detector at the near side of a videoconference, indicating various modes based on the audio signal at the far end.
- the interactivity indicator indicates interactive mode under the following conditions: 1) whenever an audio signal (the near end or the far end) exceeds a threshold and within a predetermined time period thereafter; 2) whenever both the near end signal and the far end signal exceed the threshold; 3) when condition 2 ends but within a predetermined time period. When the above three conditions are not satisfied, the interactivity indicator indicates non-interactive mode. As shown in FIG.
- the time period between time marks 1 – 1 ′, 2 – 2 ′, and 9 – 9 ′ satisfies condition 1); 3 – 4 , 5 – 6 , 7 – 8 , 10 – 11 satisfies condition 2); and 4 – 5 , 6 – 7 , 8 – 9 satisfies condition 3).
- the conference is in an interactive mode.
- the time periods between 1 ′– 2 , 2 ′– 3 , 9 ′– 10 and 11 ′ on do not satisfy any of the three conditions, and therefore are non-interactive.
- the interactive detector may also use other criteria or method to detect the different modes during a video conference for various types of video conferences.
- interactivity may be determined if near-end amplitude has exceeded threshold for at least 4 seconds of the last five, on the deduction that the far end must not be talking if the near end is talking so much.
- interactivity based on both near-end and far-end activity. For example, interactivity may be determined if near-end amplitude has exceeded threshold for 100 ms after it has not for at least 5 seconds, unless far-end interactivity has been detected within the past 5 seconds.
- Determination may be based on the relationship between such measurements made on two or more microphones at one site. These measurements may be made on the microphones individually, or as a group.
- several microphones are placed in the conference room. When the amplitudes of audio signals at several microphones exceed a threshold during any predetermined period of time, then the conference participants are interacting with each other, so the mode is interactive. If only one of the amplitudes of the audio signals from the microphones exceed the threshold during the last predetermined period of time, then only one participant is speaking, so the mode may be switched to monologue.
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